1. Field of the Invention
The present invention relates to a touch sensor attached on a flat panel display including a liquid crystal display (LCD) and an organic light-emitting diode (OLED), and more particularly, to a mutual capacitance measuring touch sensor which is insensitive to self-generated noise of a flat panel display while extracting mutual capacitance between two electrodes crossing each other in a capacitive touch sensor panel.
2. Description of the Related Art
Recently, the revolution of the IT industry has created various types of electronic devices. In particular, various electronic devices with new designs and functions have continuously emerged in the field of portable electronic devices such as notebook computers, mobile phones, and portable multimedia players (PMP).
In the case of a mobile phone or tablet PC, a touch sensor panel is attached to a flat panel display including an LCD or OLED, and used as an input device through a touch operation using a finger or pen.
As for the early touch sensor panels, a resistive touch sensor panel has been frequently used. The resistive touch sensor panel includes two flexible membranes each having a transparent electrode applied on the entire surface thereof, and the two flexible membranes are positioned close to each other with a uniform distance maintained therebetween. When a touch occurs, parts of the two flexible membranes are mechanically contacted with each other and electrically coupled to each other, and the contact position is detected through a touch sensor circuit. In this case, since the mechanical touch is transmitted to the touch sensor panel and the flat panel display through the touch operation, the lifetime of the devices may be reduced.
Recently, in order to overcome such a disadvantage, a capacitive touch sensor panel has been frequently used, which removes mechanical contact by employing tempered glass instead of flexible membranes. In the capacitive touch sensor panel, a glass plane for touch sensor panel is positioned over a flat panel display, and tempered glass is attached on the glass plane. Thus, although a touch operation is performed on the tempered glass through a finger or pen, the mechanical touch is not transmitted to the flat panel display and the glass plane for touch sensor panel, which are positioned under the tempered glass. Therefore, although a touch operation is repeatedly performed on the capacitive touch sensor panel, the lifetime of the display device is not reduced.
The glass plane for touch sensor panel may include transparent electrodes arranged thereon. The capacitive touch sensor panel is divided into a self-capacitance measuring touch sensor panel and a mutual-capacitance measuring touch sensor panel. At the beginning, the self-capacitance measuring touch sensor panel has been mainly used. However, as the number of simultaneous touches increases to three or more, the use of the mutual-capacitance measuring touch sensor panel has gradually increased.
In the capacitive touch sensor panel, a touched position may be recognized by connecting a touch sensor circuit to measure self-capacitance between each conducting wire and the ground or mutual capacitance between two conducting wires crossing each other. At this time, the reference node (ground) of the self-capacitance corresponds to an LCD common electrode (VCOM) terminal in the case of an LCD.
However, the mutual-capacitance measuring touch sensor panel has a considerably low signal-to-noise ratio (SNR), due to common electrode (VCOM) noise which is self-generated from a flat panel display such as LCD. Thus, the mutual-capacitance measuring touch sensor panel is required to reduce the influence of the VCOM noise which is self-generated from the flat panel display. Furthermore, since the mutual-capacitance measuring touch sensor panel must measure capacitance, a charge amplifier is mainly used at the first stage of a receiver circuit unit.
As the method for reducing the influence of VCOM noise which is self-generated from the flat panel display such as LCD, the following methods have been attempted in the mutual-capacitance measuring touch sensor panel:
(1) chopper method;
(2) method of increasing the amplitude of a touch sensor panel driving signal;
(3) method of adjusting the frequency of a touch sensor panel driving signal; and
(4) method of operating the touch sensor panel only during the time period in which the flat panel display is not operated.
According to the chopper method, the same signal as a driving signal applied to the capacitive touch sensor panel is applied to the receiver circuit unit, the same signal as the driving signal is passed through the charge amplifier and a chopper circuit, and an output signal thereof is passed through an integrator or low-pass filter. Then, the influence of VCOM noise in the output of the integrator or the low-pass filter may be reduced.
According to the method of increasing the amplitude of a touch sensor panel driving signal, the amplitude of the touch sensor panel driving signal may be increased in order to increase the SNR of an output signal of the receiver circuit unit to one or more.
According to the method of adjusting the frequency of a touch sensor panel driving signal, a frequency with low noise is found on the frequency spectrum of VCOM noise, and the frequency of the driving signal is adjusted to the frequency with low noise.
According to the method of operating the touch sensor panel only during the time period in which the flat panel display is not operated, since VCOM noise does not occur during VBALNK period which is required until the next frame screen is transmitted after one frame screen is transmitted in the flat panel display, the touch sensor circuit is operated only during the VBLANK period.
Before the technical idea of the present invention is described, the structure of LCD needs to be first understood. The currently used LCD may be divided into a vertical alignment (VA) LCD and an in-plane switching (IPS) LCD.
In the VA LCD as illustrated in FIG. 1, since a common electrode (VCOM) node is disposed on a top glass substrate which is positioned remote from a backlight unit of the plane LCD, between two glass substrates forming the plane LCD, the VCOM node is positioned close to a capacitive touch sensor panel electrode.
In the IPS LCD as illustrated in FIG. 2, since a VCOM node is disposed on a bottom glass substrate positioned close to a backlight unit, the VCOM node is positioned remote from a capacitive touch sensor panel electrode. In the IPS LCD, however, since no conductive plane exists between the touch sensor panel and the LCD, the touch sensor panel electrode is directly exposed to a video signal (analog gray scale signal) which is driven by a TFT or source driver.
Each pixel of the LCD includes two electrodes and liquid crystal, a color filter and the like, which are positioned between the two electrodes. The electrodes may include transparent electrodes formed of indium tin oxide (ITO) or the like over a glass plane. As illustrated in FIG. 3, an analog signal indicating gray scale is applied to one of the two electrodes through a TFT switch from the source driver, and a DC voltage of 5V is commonly applied to the other electrode in all of the pixels. This common node is referred to as a common electrode (VCOM) node. In general, the capacitive touch sensor panel has no ground or reference electrode, and is directly attached over the LCD device. Thus, the VCOM node serves as a reference voltage node of the capacitive touch sensor panel, that is, the ground.
Referring to FIG. 3, gate driver lines G1 to G3 corresponding to the respective rows or columns of the LCD are sequentially driven according to the positions thereof. Each of the gate driver lines is coupled to the gate nodes of a large number of TFT switches. In the case of full HD, about 6,000 TFT switches are coupled. Thus, a relatively large capacitance of several tens pF is coupled to one gate driver line. The gate driving signal maintains a value of about −5V during turn-off, and maintains a value of about +25V during turn-on. Thus, since a considerably large voltage variation occurs at a rising or falling edge of the gate driver signal for a short time, a considerable magnitude of displacement current (C·dV/dt) IN(t) flows into the LCD VCOM node through a gate capacitance CGD and a liquid crystal capacitance CLC of the TFT.
FIG. 4 is a diagram illustrating the mechanism in which VCOM noise is generated by a driving signal of a gate driver line of FIG. 3. Referring to FIG. 4, the displacement current IN(t) passes through a common electrode (VCOM) plane having transparent electrodes arranged thereon, and then flows through an output resistor RO of a common electrode (VCOM) driver. Thus, the waveform of VCOM noise appears in the form of an impulse at rising and falling edges of the gate driver signal.
As illustrated in FIG. 3, however, the gate driver signal is sequentially moved to the next gate driver line. In all of the gate driver lines, the VCOM noise has an impulse waveform at each of the rising and falling edges of the gate driver signal. Thus, the VCOM noise has a time-periodic waveform, and the period of the VCOM noise is equal to the time period in which the corresponding gate driver signal is maintained at a high level. The period of the VCOM noise corresponds to the half of the period of the gate driver signal.
As described above, the capacitive touch sensor panel is divided into a self-capacitance measuring touch sensor panel and a mutual-capacitance measuring touch sensor panel. When the self-capacitance measuring touch sensor panel is touched, capacitance between the human body and the earth is added to self-capacitance, thereby increasing the value of the self-capacitance. Thus, this phenomenon is used to determine whether a touch occurred. Furthermore, since the self-capacitance has a large capacitance value of 50 pF or more, the self-capacitance is insensitive to VCOM noise.
In the capacitive touch sensor panel, however, when the number of simultaneous touches increases to three or more, mutual capacitance must be measured. When a touch operation is performed, the mutual capacitance decreases. Typically, the mutual capacitance has a value of about 1 pF. As illustrated in FIG. 7 below, VCOM noise appears in an output of a charge amplifier through self-capacitance CSXj between an LCD common electrode (VCOM) and a touch sensor panel electrode coupled to the charge amplifier.
Although the amplitude of the VCOM noise is smaller than the amplitude of the touch sensor panel driving signal, the self-capacitance CSXj is 50 or more times larger than mutual capacitance CMij. Thus, in many cases, the SNR of the output signal of the charge amplifier is smaller than 1. In such a case, in order for the mutual capacitance measuring touch sensor to overcome LCD VCOM noise and to stably determine whether a touch occurred, a noise reduction-type touch sensor is required.